hyperv/kbd: Add support for synthetic keyboard.

Synthetic keyboard is the only supported keyboard on GEN2 Hyper-V.

Submitted by:   Hongjiang Zhang <honzhan microsoft com>
Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D10196

hyperv/kbd: Remove unnecessary assignment.

Reported by:    PVS
Sponsored by:   Microsoft